The present invention relates to a control system for four-wheel drive vehicles, and more particularly to a system for controlling the torque capacity of a transfer clutch for four-wheel drive vehicles.
A four-wheel drive vehicle of the type in which a transfer clutch is provided for transmitting the output torque of a transmission to auxiliary drive wheels is known. In such a vehicle, a power transmission system can be converted from a two-wheel drive system to a four-wheel drive system by engaging the transfer clutch. Meanwhile, when the vehicle negotiates corners, braking phenomenon called "tight corner braking" will occur at a sharp corner. This is caused by greater radius of turning of the front wheels than that of turning of the rear wheels and therefore tendency to rotate faster than the rear wheels, so that a large torsional torque occurs in the system. Japanese Patent Application Laid Open No. 57-84227 discloses a system which is intended to eliminate the braking phenomenon. In the disclosed system, a four-wheel drive power transmission system is converted to a two-wheel drive power transmission system when a vehicle turns a sharp corner. However, although such a system prevents the tight corner braking at sharp corners by the two-wheel drive vehicle system, characteristics of the four-wheel drive vehicle such as driveability on slippery roads, uphill driving and other conditions are not effected. On the other hand, it is known that the magnitude of the tight corner braking varies with vehicle speed and steering angle. Namely, the magnitude increases with decreasing vehicle speed and with increasing steering angle.